658
U N I T 7
Kidney and Urinary Tract Function
sphincter and the pelvic floor muscles that together con-
trol the outflow of urine. These functions are controlled
by three neurologic centers: the spinal cord reflex cen-
ters, the micturition center in the pons, and cortical and
subcortical centers.
5–7
Spinal Cord Centers
The centers for reflex control of bladder function are
located in the sacral (S1 through S4) and thoracolumbar
(T11 through L2) segments of the spinal cord (Fig. 27-2).
The parasympathetic neurons for the detrusor muscle of
the bladder are located in the sacral segments of the spi-
nal cord; their axons travel to the bladder by way of the
pelvic nerve
. The lower motor neurons for the external
sphincter also are located in the sacral segments of the
spinal cord. These neurons receive their control from the
motor cortex through the corticospinal tract and send
impulses to the external sphincter through the
pudendal
nerve
. The bladder neck and trigone area of the blad-
der, because of their different embryonic origins, receive
sympathetic outflow from the thoracolumbar (T11 to
L2) segments of the spinal cord. In the male, the seminal
vesicles, ampulla, and vas deferens also receive sympa-
thetic innervation from the thoracolumbar segments of
the cord.
The afferent input from the bladder and urethra is
carried to the central nervous system (CNS) by fibers
that travel with the parasympathetic (pelvic), somatic
(pudendal), and sympathetic (hypogastric) nerves. The
pelvic nerve carries sensory fibers from the stretch
receptors in the bladder wall, the pudendal nerve car-
ries sensory fibers from the external sphincter and pelvic
muscles, and the hypogastric nerve carries sensory fibers
from the trigone area.
Pontine Micturition Center
The immediate coordination of the normal micturi-
tion reflex occurs in the micturition center in the pons,
facilitated by descending input from the forebrain and
ascending input from the reflex centers in the spinal
cord
5
(Fig. 27-3). This center is thought to coordinate
the activity of the detrusor muscle and the external
sphincter. As bladder filling occurs, ascending spinal
afferents relay this information to the micturition cen-
ter, which also receives important descending informa-
tion from the forebrain concerning behavioral cues
for bladder emptying and urine storage. Descending
pathways from the pontine micturition center produce
coordinated inhibition or relaxation of the external
sphincter. Disruption of pontine control of micturi-
tion, as in spinal cord injury, results in uninhibited
spinal reflex–controlled contraction of the bladder
without relaxation of the external sphincter, a condi-
tion known as
detrusor–sphincter dyssynergia
(to be
discussed).
Cortical and Subcortical Centers
Cortical brain centers enable inhibition of the micturi-
tion center in the pons and conscious control of urina-
tion. Neural influences from the subcortical centers in
the basal ganglia modulate the contractile response.
They modify and delay the detrusor contractile response
during filling and then modulate the expulsive activity
of the bladder to facilitate complete emptying.
In infants and young children the pathways to the
cortical and subcortical centers are not fully developed
and micturition occurs whenever the bladder is suffi-
ciently distended. At about age 2 1/2 years it begins to
come under cortical control; in most children, complete
control is achieved by 3 years of age.
Neuromediator Control of Bladder
Function
The neuromediators for the ANS play a central role
in micturition and maintenance of continence.
6,7
Parasympathetic innervation of the bladder is medi-
ated by the neurotransmitter acetylcholine. Two types
of cholinergic receptors, nicotinic and muscarinic, affect
various aspects of micturition.
Nicotinic
(N) receptors
are found in the synapses between the preganglionic and
postganglionic neurons of the sympathetic and para-
sympathetic nervous systems, as well as motor endplates
of the striated muscle fibers of the external sphincter
and pelvic muscles.
Muscarinic
(M) receptors are found
in the postganglionic parasympathetic endings of the
detrusor muscle. Several subtypes of M receptors have
been identified. Both M
2
and M
3
receptors appear to
mediate detrusor muscle activity, with the M
3
subtype
mediating direct activation of detrusor muscle con-
traction. The M
2
subtype appears to act indirectly by
inhibiting sympathetically mediated detrusor muscle
relaxation.
5–7
The identification of muscarinic receptor
subtypes has facilitated the development of medications
T11
T12
L1
L2
Sympathetic
neurons
Somatic
pudendal
nerve
Internal sphincter
External sphincter
Detrusor muscle
Parasympathetic
pelvic nerve
Afferent fibers
Efferent motor
Efferent inhibitory
S1
S2
S3
Sympathetic
hypoglossal
nerve
S2
S3
S4
FIGURE 27-2.
Nerve supply to the bladder and the urethra.
